A spark plug for providing ignition in an internal combustion engine, such as an automobile engine, generally includes a tubular metallic shell; a tubular insulator disposed in a hole of the metallic shell; a center electrode disposed at the forward side of an axial hole of the insulator; a metal terminal member disposed at the rear side of the axial hole; and a ground electrode whose one end is joined to the forward end of the metallic shell and whose other end faces the center electrode and forms a spark discharge gap in cooperation with the center electrode. Furthermore, in order to prevent generation of radio noise, a known spark plug has a resistor provided in the axial hole between the center electrode and the metal terminal member.
In recent years, high output and high efficiency have been required of internal combustion engines of automobiles, etc.; in this connection, in order to attain free engine design, a reduction in engine size, etc., demand has been rising for development of a small-sized spark plug. In order to reduce the size of a spark plug, reducing the diameter of a hole in the insulator is inevitable. However, in some cases, a conventionally designed spark plug has involved a deterioration in under-load life as a result of the insulator being reduced in size.
In order to cope with such a problem, for example, claim 1 in Patent Document 1 provides “a spark plug characterized in that . . . the electrically conductive glass seal layer has a diameter D of 3.3 mm or less, and a joint surface between the resistor and the electrically conductive glass seal layer is curved.” The document describes that the invention “can provide a spark plug whose diameter is reduced and which exhibits excellent vibration resistance and under-load life of the resistor, through enhancement of adhesion between the resistor and the electrically conductive glass seal layer” (refer to Paragraph No. 0012).